This invention relates to a suspension system for a vehicle and more particularly to a suspension system which comprises a shock absorber of damping force characteristic variable type provided between a sprung member and an unsprung member.
In general, the vehicle suspension system comprises a shock absorber, for damping the oscillation of a wheel, interposed between a sprung member and an unsprung member. There are various types of shock absorbers and shock absorbers of damping force characteristic variable type are grouped into one which damping force characteristic (characteristic of different damping coefficient) is changed into two levels (higher level and lower level) and the another which the damping force characteristic is changed into many levels or changed steplessly.
The basic function of the shock absorber of damping force characteristic variable type is that the damping force characteristic of a shock absorber is set lower (i.e., SOFT side) when the damping force generated by a shock absorber acts in the oscillation-stimulating direction (in the same vertical direction as a sprung member) and the damping force is set higher (i.e., HARD side) when the damping force acts in the oscillation-restraining direction (in the opposite vertical direction to the sprung member) so that oscillation-restraining energy is set larger than oscillation-stimulating energy. Thus, comfortableness to ride in and running stability is improved.
Various methods for judging in which direction the damping force of a shock absorber is acting, either in the oscillation-stimulating direction or oscillation-restraining direction with respect to the sprung member, are proposed. Fir example, the Japanese Patent Application Laying Open Gazette No. 60-248419 discloses a method that judges a direction of a damping force in such a way: the sign of relative displacement between the sprung member and the unsprung member is checked whether it is same as a sign of differentiated value of it, which is relative speed between the sprung member and the unsprung member. If signs are same, it is observed that damping force is acting in the oscillation-stimulating direction and if signs are not same, it is observed that damping force is acting in the oscillation-restraining direction. Also, the Japanese Patent Application Laying Open Gazette No. 61-163011 discloses a method, in which a sign of absolute speed of the sprung member is checked whether it is same as a sign of relative speed between the sprung member and the unsprung member. If signs are same, it is observed that damping force is acting in the oscillation-restraining direction and if signs are not same, it is observed that damping force is acting in the oscillation-stimulating direction.
However, when the sprung member is in a high oscillation frequency range due to bounce and recession of a road, the sign of relative displacement between the sprung member and the unsprung member, the sign of relative speed between the sprung member and the unsprung member, and the sign of absolute speed of the sprung member change constantly. Therefore, conventional control method possesses problems such as loud noise or oscillation caused by unnecessary switching of the damping force characteristic of the shock absorber. Also, when the damping force characteristic of each shock absorber is switched to higher level, the oscillation of the unsprung member caused by the bounce and recession on a road is transmitted to the sprung member easily and accordingly, a passenger feel unsmooth and uncomfortable ride.